Fig. 1 HRTEM micrographs and images of sulfated ZrO2 nanocrystals embedded in nanotubes of SBA—15 mesoporous silica in sample SO4/60%ZrO2/SBA:
left—side view; right—front view (channel entrances).
overnight and calcined at 873 K for 3 h (heating rate 15 K
min21).The content of the ZrO2-T phase (crystal size ~ 3.5 nm)
detected in calcined supported SZ by XRD was equal to its
potential content (EDX) up to the S+Zr in the initial slurry =
0.3. It yielded a S+Zr ratio of = 0.15 in calcined SZ-SBA-15
composites. At higher initial S+Zr ratios the crystallinity of the
calcined zirconia phase decreased due to formation of Zr(SO4)2
being 60 and 12% at S+Zr = 1.0 and 2.0, respectively. The
reference SZ was prepared according to ref. 3, starting from
ZrOCl2 with the final calcination temperature of 873 K. The
material contained ZrO2(T) phase having a crystal size of ~ 11
nm (XRD) and S+Zr ratio of 0.037 (Table 1). Similar
characteristics figures were displayed by the commercial SZ
sample (Table 1). The maximal S+Zr ratio obtained in calcined
SZ-SBA-15 composites at 100% ZrO2-T crystallinity was ~ 3
times higher than in bulk SZ samples being inversely propor-
tional to the crystal size of ZrO2(T)-phase.
crystallinity of ZrO2(T) phase. The performance evaluation was
carried out in two selected acid-catalyzed reactions generally
used for characterization of SZ performance: condensation of t-
BuOH and MeOH (Reaction 1)† and dehydration of iso-PrOH
(Reaction 2).† As expected, the activity of SZ-SBA-15
composites was 1.5–2.2 times higher compared with bulk SZ.
The proportionality of substrate conversions to the catalysts
sulfur content and similar selectivities to MTBE (100%) and
diisopropyl ether measured with supported and bulk SZ are
evident for the same nature of acid sites in the both the bulk and
supported systems. Increasing the sulfur content in calcined SZ-
SBA-15 composite beyond the optimal S+Zr value of ~ 0.15
caused a substantial drop of catalytic activity due to decreased
crystallinity of ZrO2(T) phase.
This research was supported by the Israel Academy of
Sciences and Humanities. The authors thank to Mr V.Ezersky
for conducting the HRTEM measurements.
At this optimal S+Zr ratio the SZ-phase was uniformly
distributed inside the nanotubes of SBA-15 as 2–4 nm
nanoparticles (Fig. 1) leaving ~ 3.5 nm mesopores and surface
area of 280–405 m2 g21 depending on Zr-loading (Table 1). The
high values (0.83–0.92) of normalized surface area calculated
for calcined SZ-SBA-15 composites,14 indicate minimal pore
blocking of parent MS. Parallel fringes across the nanoparticle
images in the insets A,B of Fig. 1 have a periodicity of 3.0 Å
which corresponds to planes (101) with d-spacings equal to d101
= 3.00 Å in tetragonal ZrO2 structure. An electron diffraction
pattern at the inset C (Fig. 1) exhibits a set of diffraction spots
which could be indexed on the basis of unit cell parameters of
ZrO2-T phase.
Notes and references
† Reaction 1 was conducted at 398 K, MeOH/t-BuOHmol = 2.4, t = 2 h,
catalysts loading 8.9 mg ml21; Reaction 2—at 473 K, t = 2 h, catalysts
loading 3 mg ml21
.
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The amount of sulfur species adsorbed by zirconia nano-
crystals embedded in SBA-15 at 100% ZrO2(T) crystallinity
was 3 times higher than in a regular bulk SZ (Table 1).
Assuming them to be acid sites-creative species one could
expect a 1.5–2 times higher acidity of optimized SZ-SBA-15
composites with 48–60 wt% ZrO2 loading relative to the bulk
SZ. TPD (AMI-100,Zeton-Altamira) detected 1.5–2.1 times
higher the amount of NH3 desorbed from ZrO2-SBA-15 at
393–1073 K after saturation at 313 K compared with that
desorbed from reference SZ samples.
The catalysis tests were performed in a 50 cm3 batch reactor
at conditions where no transport limitations affected the
reactions rates. This was proven by the linear dependence of
substrate conversion on the catalysts loading (mg ml21
reagents).
Table 1 presents the testing results for bulk and supported SZ
catalysts optimized according to the sulfur content at 100%
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